Recirculating systems are expected to play a key role in the expansion of US aquaculture production. Recirculating systems allow year-around production of aquatic organisms under controlled conditions, require very little water and land, have minimal effluent discharge, and can be set up almost anywhere, including in cities close to major markets. These systems can also be operated in a biosecure manner unlike other forms of aquaculture. Recirculation aquaculture is currently not economically favorable, primarily because of the high costs of building and operating the complex systems required for nitrogenous waste removal (biofiltration). We propose to develop and test a simple and efficient device that converts ammonia in water directly into nitrogen gas using patented nanotechnology procedures, and a process called photoelectrocatalytic oxidation (PECO). Several variables need to be evaluated and optimized before a commercial-scale device can be used to replace biological filtration in closed-system aquaculture. Phase 1 experiments will optimize PECO reaction kinetics by evaluating the effects of photoanode chemistry, light quality, applied voltage, pH, and salinity on the rate of ammonia oxidation and byproduct formation (e.g., nitrite, nitrate). Phase 2 will focus on designing, fabricating and testing a commercial-scale unit for removing ammonia from tanks stocked with fish. Additional markets for our device include the aquarium industry, waste water treatment, water bottling, and drinking water supplies, where this technology should assist in removing organic contaminants (e.g., endocrine disruptors), disease-causing microorganisms (E. coli), and potential biological and chemical threat agents (e.g., G- and V-series nerve agents, brucellosis, ricin).